Antimony containing metal oxide catalysts and, particularly, metal oxide compositions containing, as essential components, (A) antimony, (B) at least one element selected from the group consisting of iron, cobalt, nickel, manganese, uranium, cerium, tin and copper, (C) at least one element selected from the group consisting of vanadium, molybdenum and tungsten, and (D) tellurium, are known and are used for production of unsaturated aldehydes by oxidation of olefins, unsaturated nitriles by ammoxidation of olefins and diolefins by oxidative dehydrogenation of olefins, etc. For example, such catalysts are disclosed in U.S. Pat. No. 3,668,147, Japanese Patent Publications Nos. 19764/72, 40957/72 and 40958/72, U.S. Pat. No. 3,716,496, Japanese Patent Publications 19766/72 and 19767/72, and U.S. Pat. No. 3,988,359, etc.
In spite of their good catalytic performance, none of the above catalysts are fully satisfactory after prolonged use and their service life is not always sufficiently long. Even the activity of improved catalysts gradually decreases with extended use and improper reaction conditions often accelerate reduction in catalytic activity. It is economically unfeasible to continue using a catalyst whose activity is reduced below a certain level. In particular, when the catalyst is used industrially on the large scale as in the preparation of acrylonitrile, the influence of the deterioration is large and when the deteriorated catalyst is not replaced with fresh catalyst at the appropriate time, economical loss occurs to a remarkable extent. However, since catalysts of the above specified type are expensive, replacement of the deteriorated catalyst with fresh catalyst is a substantial expenditure. It would, therefore, be economically advantageous if a practical method for regenerating the catalyst were available.
As can be understood from the above explanation, one criterion for determining whether a catalyst is deteriorated or whether a deteriorated catalyst has been regenerated by a regeneration method is economically feasible, in contrast to a technically feasible, which takes into account the activity and selectivity of the catalyst. Based on experience a catalyst is considered "deteriorated" if the yield of the end product is reduced by more than 2 to 3% of the yield obtained using fresh catalyst, and a catalyst is considered "regenerated" if such yield is restored to the original yield level or higher.
It is difficult to enumerate the causes of catalyst deterioration which occurs during catalyst use. In most cases, many factors combine to cause such deterioration, and what is more, locating a particular contributing factor does not directly lead to the development of an effective method of regenerating the catalyst. Therefore, many attempts at providing effective catalyst regeneration have not been successful.
A method of regenerating an antimony-uranium oxide catalyst is described in U.S. patent application serial Nos. 83,187, and 103,005 (corresponding to Japanese Patent Application (OPI) No. 8615/72) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and British Patent No. 1,365,096. That method comprises heating an antimony-uranium oxide catalyst complex in a fluidized state, in a non-reducing gas at a temperature of from 800.degree. to 18OO.degree. F. and for a time such that the surface area of the catalyst does not fall below a minimum critical level of 5 m.sup.2 /g. The basic concept behind the method is to heat the catalyst before the catalyst performance, which is determined by the surface area of the catalyst, drops to a minimum critical level. In addition, the method is applicable over a wide range of temperature. Therefore, this method may be considered a satisfactorily practical method for regenerating such a catalyst. However, antimony containing oxide catalysts where antimony is combined with elements other than uranium cannot be regenerated using such a simple procedure. For example, U.S. Pat. No. 4,208,303 discloses that iron-antimony containing oxide catalysts which have a specific composition can only be regenerated under relatively limited conditions and only when the deteriorated catalyst has a specified nature. Accordingly, the method described in Japanese Patent Application (OPI) No. 8615/72 is only suitable for regenerating of an antimony-uranium oxide catalyst.
U.S. Pat. No. 4,049,575 discloses a process for the production and improvement of catalysts including a catalyst which may be regenerated in accordance with the process of this invention. In accordance with the process disclosed in U.S. Pat. No. 4,049,575 a catalyst composition is prepared by impregnating or spraying onto a mixed metal oxide composition consisting of antimony and a specific metal with a solution containing other active components. The process can be advantageously used in regenerating a deteriorated catalyst as demonstrated by the specific examples in U.S. Pat. No. 4,049,575, but the method is rather complex and costly because it involves preparing a solution of the catalytic component with which the catalyst is impregnated, impregnating the catalyst with a predetermined amount of the solution, drying, and calcining the impregnated catalyst. In particular, the method requires that the impregnating solution contain at least two catalytically active components, but it is not easy to prepare one stable impregnating solution which does not produce a precipitate, for instance. As a result, it is sometimes necessary to use rather expensive reagents as starting materials for the active components of the catalyst. The method also introduces new catalytic components to the catalyst, thus yielding a regenerated catalyst having a different composition and different physical properties than those of the original catalyst or having a different reaction rate and different optimum reaction conditions. Therefore, it is often difficult to use a catalyst regenerated in this way in combination with fresh (unregenerated) catalyst without some disadvantages occurring.
Further, Japanese Patent Application (OPI) No. 81191/79 (corresponding to U.S. patent application serial No. 959,810, field Nov. 18, 1978) provides a process for regenerating an antimony containing oxide catalyst, which comprises impregnating or spraying onto the deteriorated antimony containing oxide catalyst an aqueous solution of nitric acid and/or a nitrate and then drying the impregnated metal oxide catalyst followed by calcining the impregnated catalyst at a temperature ranging from 400.degree. to 1000.degree. C. The method disclosed in Japanese Patent Application (OPI) No. 81191/79 can be conducted with a wide range of catalysts to be regenerated and is a comparatively simple process from the standpoint of the regeneration operations and conditions. However, because nitric acid and/or the nitrate is very corrosive, the materials which can be used for the regenerating apparatus are extremely restricted and since large amounts of a nitric acid and nitrogen oxide are present in the waste gas, the waste gas cannot be vented to the outside, the process has the disadvantage that the apparatus for treating the waste gas must be equipped with pollution control devices to avoid pollution problems. Therefore, while the method is comparatively simple to conduct, the method has economical problems associated with its industrial use. On the other hand, in order to overcome the above problems with respect to the regeneration of the catalyst, this invention was achieved.